DIGHTON — The shark in the Taunton River early Wednesday night was big but not nearly as fearsome as some initially thought.

Dighton Harbormaster Ronald Marino said he jumped into his 17-foot powerboat after getting a call at 6:30 p.m. from Dighton police that a shark had been sighted near the water’s surface.

Marino, who has been harbormaster for 20 years, said he was able to get fairly close to the big fish, which he estimated was 10 to 12 feet long. He put its location at 100 yards west of red nun buoy No. 20.

After conducting some Internet research, he concluded it had been a sand tiger shark.

Marino speculated the shark, which reaches a length of about 10 feet, had been attracted by schools of bait fish in the river.

“He was flailing around for about half an hour — I think he got caught on a sandbar,” Marino said.

When the tide came in, he said the shark managed to free itself and swim into the river’s channel. But Marino said he couldn’t tell which way the fish went.

“He went under, and that’s the last we saw of him,” said Marino, who also noted he’d spoken last week to Somerset’s harbormaster — who informed him of a report a shark in the section of Taunton River that flows past his town.

But Thursday afternoon, after examining photos taken by Marino and his son, Mark, a renowned biologist came to an altogether different conclusion.

“There’s no way it was a sand tiger. It was a basking shark,” said Gregory Skomal, a senior marine fisheries biologist with the Massachusetts Division of Marine Fisheries.

Skomal said the shape and size of the tail and dorsal fins were a dead giveaway for what is the second-largest fish in the world, second only to the whale shark.

Basking, whale and megamouth sharks are all plankton-eating fish. They are also recognized as docile and nonthreatening to humans.

Basking sharks are known to grow to more than 30 feet long, which is why Skomal said the one that visited Dighton was a juvenile.

“I’d say it was a small one. They don’t mature until they hit 18 or 19 feet,” he said.

Skomal said the plankton-eating traveler likely wandered up river after being lured by “a big plankton bloom.”

Page 2 of 2 - Basking sharks, he said, are known to be seasonal visitors to parts of the Massachusetts coastline, usually from May through September.

Despite crossing into warmer waters, sometimes as far south as the equator, they tend to frequent deep, northern waters in search of plankton concentrations.

“They’re not really capable of biting,” said Skomal, who described the passive-feeding, basking shark’s teeth as similar to “a grain of rice cut in half.”

“They’re very whalelike. It’s certainly not a predator,” he said.

Skomal said he’s concerned that the shark in the Taunton River might turn up dead. But he also said it’s a good sign it swam into the river’s channel when the tide rose.

The hope, he said, is that it headed downstream back toward Mount Hope Bay, as opposed to upstream where the freshwater supply can be lethal.

Skomal said this is the first time he’s heard of a shark sighting in the Taunton River as far north as Dighton, which borders Taunton.

The basking shark spotted on Wednesday, he said, “must have zigged when it should have zagged.”

Skomal also said he hadn’t been aware of reports in June of a beluga whale spotted more than once in the Taunton River in Fall River.

He said he preferred not to opine on the subject since he doesn’t consider himself an expert on whales. But Skomal nonetheless said he was surprised to hear the news.